The present invention relates to adjusting the motor performance of a scroll compressor to selectively provide maximum efficiency and maximum torque.
A scroll compressor generally includes a pair of interfitting scroll wraps each connected to a planar base. One of the wraps is non-orbiting and the other wrap orbits relative to the non-orbiting wrap. The wraps are in contact with each other and define compressive chambers for an entrapped fluid. As the orbiting scroll moves relative to non-orbiting scroll, the size of the compression chambers change to compress the fluid.
Scroll compressors are widely utilized in many compression applications. Scroll compressors have high efficiency and thus are increasing in popularity. However, there are some challenges during scroll compressor operation.
An electric motor is commonly provided to drive compressors including most scroll compressors. The motor must typically produce a greater quantity of torque for some operating conditions than is required for other operating conditions. The need to provide a wide range of output torques often requires an undesirable compromise in the motor performance characteristics over its range of operation.
One known drive motor is a phase split capacitor (PSC) single phase motor. A main winding and an auxiliary winding each have an input lead while also sharing a common lead. The main winding input lead supplies electric current to the main winding which also passes through the common output lead. The auxiliary winding input lead includes a capacitor prior to the auxiliary winding. The current passes through the capacitor, through the auxiliary winding and also through the common lead. The capacitor provides an electrical phase offset between main winding and auxiliary winding currents to allow self-starting of the motor and to augment its performance when running under load. The number of turns of wire in the main and auxiliary windings determine the performance characteristics of the motor.
A typical performance curve for a known PSC single phase motor is defined by plotting the motor efficiency E as a function of motor output torque. Two defining points on the E curve are the maximum torque point M and the rating point R. The maximum torque output M of the known PSC motor is specified so that the motor will not stall when running at peak load. The known PSC motor is designed to deliver maximum output and to deliver the highest possible efficiency at rating point R. Often, these two load points are far apart. It is therefore not possible to both achieve maximum efficiency at load point R while still being able to run at maximum load point M. In other words, the peak of the efficiency curve E falls somewhere between rating point R and maximum load point M. Thus the efficiency of the motor at the rating point R must be compromised to assure that the motor will deliver maximum torque M when needed.
The efficiency of the motor at rating point R can be increased by adding turns to main winding and auxiliary windings, and, optionally, increasing the capacitance of the capacitor. However, this also weakens the motor and reduces the torque output at the maximum load point.
Accordingly, it is desirable to provide a motor with multiple windings which can be selectively connected to provide maximum efficiency at rating point R or reconnected to provide maximum torque output at maximum load point M as required.
The compressor system according to the present invention generally includes a motor mounted within a housing and a pump unit. The pump unit is preferably a scroll compressor. A power supply supplies the phase power to the motor.
In one embodiment of the present invention, a single phase PSC motor includes a switch assembly that includes a double pole single throw type switch in which alternate leads are individually connected to separate poles on one side of the switch. During normal load operation, when maximum efficiency is desired, the switch is placed in the open position. Current passes through first and second main windings and on to the common lead. Current also passes through a first auxiliary capacitor and a first auxiliary winding connected in parallel with the first and second main windings.
During high load operation, when maximum torque output is needed at a load point, the switch is placed in the closed position. Current, which formerly flowed through the second main winding, is now bypassed through the alternate lead, the switch, and out through common lead. Current is also passed through a second auxiliary capacitor and a second auxiliary winding in parallel with the first auxiliary capacitor and the first auxiliary winding. In effect, this reduces the number of main winding turns in the motor and thus strengthens it. The fact that the auxiliary windings are connected in parallel also has the net effect of reducing the auxiliary winding inductance, which has the same net effect as if turns were removed from the auxiliary winding without adding in the second auxiliary lead with the auxiliary winding into the circuit. That is, during normal steady operation, when high load is required, the switch is closed.
As the auxiliary capacitors are also now connected in parallel, the net effect is an increase in the total auxiliary capacitance, which also serves to strengthen the motor. The division between the number of turns in the first and second main windings, the number of turns in the second auxiliary winding, and the capacitance of second auxiliary capacitor are all preferably selected to achieve maximum torque output at the maximum load point.
Another embodiment of the present invention provides a three-phase WYE connected motor. Input leads are each connected to a separate leg of a three phase power supply. During normal load operation, a first set of main windings, a second set of main windings and a third set of main windings are connected in parallel to a common pole. Each set of windings includes a first and a second winding connected in series. The number of turns in the first set of main windings, the second set of main windings and the third set of main windings are all selected to provide high efficiency at a load point.
During high load operation, when maximum torque output is needed at a load point, the switch is placed in the closed position. Current, which formerly flowed through the second main winding, is now bypassed through the alternate lead, the switch, and out through common lead. Current is also passed through a second auxiliary capacitor and a second auxiliary winding in parallel with the first auxiliary capacitor and the first auxiliary winding. In effect, this reduces the number of main winding turns in the motor and thus strengthens it. The fact that the auxiliary windings are connected in parallel also has the net effect of reducing the auxiliary winding inductance, which has the same net effect as if turns were removed from the auxiliary winding without adding in the second auxiliary lead with the auxiliary winding into the circuit. That is, the switch is closed while the motor is running and when high load is required.
In yet another embodiment, a switch for a three phase delta connected motor is provided. Input leads are each connected to a separate leg of a three phase power supply. Each main winding is split into two windings in series. Leads tap between each of the respective pairs of coils and run to a switch assembly which preferably includes a single throw triple pole switch. Each lead is connected to one of the three poles of the switch.
When the switch is in the open position, the motor operates as a delta connected motor with all coils energized, that is, it is configured for high efficiency. When the switch is in the closed position, the motor is configured for maximum torque output at a maximum load point.
Preferably the switch assemblies communicate with a controller and the motor. The controller responds to any number of input variables to control the operation and timing of the switching. Such input variable can include current through the motor, motor temperature, pump temperature, pump pressure inputs, and others.